The development in the switch device of the power semiconductor promotes the devolvement in the technology of the power electronic converter device rapidly, such that various types of converter device emerge with the Pulse-width modulation (PWM for short) Technology. The active front end has been widely used in the field of power electronic technology, having the high power factor and the low Total Harmonic Distortion (THD for short).
Currently, a three-phase PWM rectifier is a common active front end in the market. The three-phase PWM rectifier includes three parallel single-phase bridge-arms, and each set of them has a number of switch elements. When the switch elements in the upper and lower bridge-arms are turned off simultaneously, the phenomena of break-though may occur which results in damage to elements of the rectifier easily. With many elements, complicate structure and high price, the three-phase PWM rectifier is often used in the energy feedback application.
The three-phase PWM rectifier may cause switch order harmonic voltage and current during performing switch action, which pollutes the gird. Thus, elimination of current harmonics in the three-phase PWM rectifier absorbs the concern in the industry.
In order to satisfy the characteristic of the three-phase PWM rectifier requiring a passive device to store energy in operation and reduce harmonic component of the current into the gird in proximity to switch frequency of the three-phase PWM rectifier, the passive device is required to be serially connected between the three-phase PWM rectifier and grid for filtering.
FIG. 1 is a diagram of a circuit in which a three-phase PWM rectification unit with inductor L filtering is connected with grid in the art. The three-phase PWM rectification unit 2 is coupled with grid through three sets of inductor L filter unit 1, and converts AC input voltage with fixed operation frequency on grid to DC output voltage. On the one hand, the inductor L in the inductor L filter unit 1 can function as an element of storing energy which makes the three-phase PWM rectification unit 2 having operation nature of Boost, and on the other hand, the inductor L can function as a filter unit for filtering switch harmonic.
When the inductor L is selected as a filter unit, harmonic is attenuated at a rate of −20 dB/Dec for all range of frequencies, thus the rate is low. However, in the case of high power application, since switch frequency is lower, a higher rate of attenuation of harmonic is required, thus it is required to add inductance to achieve better filtering effect for switch harmonic. As known, increasing inductance can prolong the response time of the entire system so as to reduce dynamic performance of the system, and the volume and the cost of the inductor L may greatly increase along with the increase of power of the active rectification unit, which limits its application in the case of large power.
Therefore, it is an urgent issue how to develop a three-phase rectification module capable of applying in the case of medium and high voltage and large power, small amount of inductance used, fewer switching devices, simple structure, anti-break-though capability, and low price.